In recent years, fiber-optic chemical sensors have been developed to detect the presence, and monitor the concentration of, various analytes. These analytes include oxygen, carbon dioxide, glucose, inorganic ions, and hydrogen ions, in both liquids and gases. These sensors rely on the recognized phenomenon that the absorbance, and in some cases the luminescence, of certain indicators is specifically perturbed in the presence of certain analytes. The perturbation of the luminescence and/or absorbance profile of the indicator can be detected by monitoring radiation that is absorbed, reflected, or emitted by the indicator when it is illuminated in the presence of a specific analyte. Thus, fiber-optic probes have been developed that position an analyte-sensitive indicator in a light path. These probes may include a pair of optical fibers. One fiber transmits electromagnetic radiation from a light source to the indicator; the other fiber transmits the return light from the indicator to a light sensor for measurement. Typically, the indicator is housed in a sealed chamber with walls permeable to the analyte. Alternatively, the probe may comprise only one optical fiber. An example of a single fiber-optic physiological probe may be found in U.S. Pat. No. 4,925,268, which is hereby fully incorporated by reference.
A fiber-optic pH probe may include an ion-permeable membrane envelope that encloses the distal ends of a pair of optical fibers. The envelope is a short section of dialysis-type tubing that fits closely about the two fibers. A pH-indicating dye-containing solid material (e.g., phenol red/methyl methacrylate copolymer microbeads) is packed tightly within the membrane distal to the ends of the fibers. A cement is applied to seal the distal end of the membrane, and also the proximal end, where the optical fibers enter the membrane. The membrane has pores sized to allow passage of hydrogen ions, but sufficiently small to preclude passage of dye-containing solid material. During operation, the probe optically detects the change in color of the pH-sensitive dye by monitoring the intensity of light reflected or absorbed by the dye at a specific wavelength. One of the fibers is connected at its proximal end to a light source, while the other fiber is connected at its proximal end to a light sensor. Light is backscattered through the dye from one fiber into the other fiber. In preparing the dye-containing material, light-scattering polystyrene microspheres of about 1 micron diameter may be added, prior to incorporation of the dye material into the hollow membrane. Fiber-optic probes for analytes other than pH can be constructed similarly using appropriate indicator compositions that undergo a change in absorbance or luminescence that is detectable by a light sensor.
As previously indicated, fiber-optic chemical sensors can be made using a single optical fiber. Single-fiber sensors reduce production costs and those coated with an indicator-containing matrix reduce calibration problems significantly. Such single-fiber probes are also small enough to pass through a hypodermic needle, and flexible enough to be threaded through blood vessels for physiological studies. The '268 patent discloses a drift-free fiber-optic sensor that has an analyte-permeable matrix disposed in the light path defined by the axial core at one end of a single optical fiber segment. The matrix contains an indicator molecule covalently linked to a preferred copolymer selected from the following: methyl methacrylate/methacrylamidopropyltrimethylammonium chloride, N-vinylpyrrolidone/p-aminostyrene, methyl methacrylate/hydroxymethyl methacrylate, methyl methacrylate/N-vinylpyrrolidone, or methyl methacrylate/acrylic acid. It is further disclosed that these polymers are preferably formulated in the range of from about 60:40 to about 80:20 by wt. %. Further, drift-free performance is obtained with sensors having matrices of these polymers less than 70 microns in thickness.
In order to be commercially useful as a probe or sensor, the device must pass a timed test. Thus, a pH probe, for instance, must reach 90% of the true or equilibrium pH value at a predetermined time, t90%, or fail and be rejected. Whereas the fiber-optic sensors of the '268 patent are represented as solving the problems of calibration and reducing manufacturing cost, the '268 patent does not address the issue of response time and that copolymeric matrices are found to have a relatively high, and unpredictable, failure rate that varies from one batch of matrices to the next. Thus, it is desirable to develop fiber-optic sensors and probes that have reduced failure rates; less variation from one matrix batch to another matrix batch; and reduced t90% response times.